The invention relates to a water-borne resin composition used to treat fibrous substrates to improve their strength under both ambient as well as wet or humid conditions, and more particularly to a paper substrate treated with a resin composition, which treated substrate is preferably capable of being both repulped and recycled.
Fibrous substrates, including corrugated and non-corrugated paperboards, papers, and other cellulosic substrates are useful for an extremely wide variety of applications, but particularly for making containers such as packaging and shipping containers. Other products include towels, tissues, spiral cans, folding carton stock, molded pulp products, and the like.
Typical processes for forming corrugated materials are well known. For example, a corrugated board may be formed on a corrugator where large rolls of linerboard paper and a large roll of the medium paper (the starting raw material for forming the corrugated core layer) will be positioned at the upstream end of the corrugator. The process normally has the medium passing through a set of corrugating rolls and thereafter being bonded to the first liner as it travels in a downstream direction. A suitable adhesive is utilized to attach the flute tips of the corrugated medium to the inside surface of the first liner.
Thereafter, at a downstream location, the other liner material is bonded to opposing flute tips of the corrugated medium with a suitable adhesive to form the combined board. Thereafter the combined board travels into a heating section to allow the adhesive to cure fully and to bond the liners to the fluted (corrugated) medium. Thereafter, slitter-scorer and cutoff devices function to produce individual sheets of slit and scored blanks for converting into containers.
As is well recognized by those skilled in the art, the structural requirements for combined board are determined by the particular requirements of the end use. Standards have been developed over the years, and both the liner and medium materials are manufactured to preselected basis weights with the end-use requirements in mind. In a container, a particularly important property is vertical stacking strength (top to bottom) where the vertical walls in a container are expected to support in-use compression loadings. In almost all packaging end uses, the fluted medium will be oriented in a vertical direction in the container side walls. In this orientation the liners and fluted medium will provide good vertical stacking strength. The vertical stacking strength for a container depends to a large extent upon the basis weights of the component materials. Using higher basis weights for the component materials results in higher vertical stacking strengths and normally greater top-to-bottom compression resistance. Obviously, in order to create the higher basis weights, additional fiber must be utilized which adds to the cost of producing the combined board and the resulting container.
Ways have been sought for many years to increase stacking strength and/or crush resistance of corrugated containers without the need for using additional fiber (higher basis weight). Various stiffening agents, such as thermosetting polymers, have been sprayed or coated onto one or both liner sheets, or onto the medium, at some point in the manufacturing process. However, many such thermosetting resins require the use of heat to cure them, and when cured, they may produce a carton or container blank which is brittle and difficult to fold. There may also be environmental problems if solvents are used in conjunction with the application of such thermosetting resins or if there is off-gassing of reaction by-products.
Another drawback to many fiberboards, including both corrugated and non-corrugated paperboard, is their poor rigidity when exposed to humid or wet conditions. To overcome this shortcoming, manufacturers have tried various ways of reinforcing fiberboard and/or improving the water or moisture resistance of the fiberboard. Examples of these attempts include impregnating or coating the fiberboard with paraffin waxes (including hot melts) or other polymeric materials.
Paraffin wax coatings substantially decrease the tendency of the fiberboard to absorb water, making paraffin-reinforced corrugated paperboard popular for use in packaging produce, poultry, and meats. Unfortunately, paraffin has several disadvantages, including being readily softened by moderately elevated temperatures. Also, it renders the container non-recyclable and non-repulpable because of the difficulties encountered in attempting to separate the wax coating from the cellulosic fiber using conventional equipment. In addition, paraffin tends to melt into other liners, causing unsightly grease and stain marks. Finally, paraffin waxes are flammable, and thus introduce safety concerns.
In view of the shortcomings of reinforcing fiberboard using paraffin, other polymeric resins, particularly various thermoset materials (i.e., materials which do not soften after cure), have been considered for this purpose. Many cured thermosets have the advantage of being very rigid. As a result, fiberboards reinforced with cured thermosets tend to have high resistance to compression. Unfortunately, many currently favored thermosets are extremely brittle after being fully cured and fracture when subsequently creased or folded. Such fracturing of the thermoset reinforcing agent can readily extend to the fiberboard itself, thereby seriously reducing the integrity of the container made therefrom along edges and at corners.
Examples of such thermosets include phenolic resins which have been applied as 100%-solids liquid solutions and then cured. Representative U.S. Pat. Nos. disclosing use of phenolic resins include U.S. Pat. Nos. 3,886,019, 4,096,935, 4,051,277, and 4,096,305 to Wilkenson et al. These patents disclose the application of thin films of phenolic resins to surfaces of linerboards and corrugated medium that will be adhered together to form the corrugated paperboard. After adhering together the linerboards and corrugated medium, the corrugated paperboard can be cut, scored, and slotted to make box blanks. Because of the brittleness of the fully cured treated board, full curing of the resin is delayed until after the box blanks have been folded to make cartons.
Various thermoset blends of phenolic resins with other resins have also been tried in an attempt to reduce the brittleness of phenolic resin alone. Representative U.S. patents include Reisman et al U.S. Pat. No. 3,687,767 (phenol-aldehyde); LeBlanc et al., U.S. Pat. No. 3,607,598 (phenol-aldehyde plus polyvinyl alcohol); Reisman, U.S. Pat. No. 3,616,163 (phenol-aldehyde resole); Elmer, U.S. Pat. No. 3,619,341 (phenol-aldehyde resole); Burke, U.S. Pat. No. 3,619,342 (phenol-aldehyde resole); Reisman et al., U.S. Pat. No. 3,697,365 (resole phenolic plus an organosilyl compound); LeBlanc, U.S. Pat. No. 3,682,762 (resole phenolic plus polyaminoalkyl substituted organosiloxane); LeBlanc, U.S. Pat. No. 3,617,427 (aminoplast-modified phenol-aldehyde resole); Carlson, U.S. Pat. No. 3,617,428 (aminoplast with phenol-aldehyde resole); and LeBlanc, U.S. Pat. No. 3,617,429 (aminoplast plus phenol-aldehyde and polyvinyl alcohol). Other reinforcing materials which have been utilized in this art include polyether materials (Tiedeman, U.S. Pat. No. 5,545,449) and isocyanate resins (Wallick, U.S. Pat. No. 5,332,458). However, brittleness and high cost problems remain. Further, such reinforcing materials render the containers non-recyclable and non-repulpable because of difficulties in separating the reinforcing materials from the cellulosic fibers using conventional equipment.
Accordingly, there remains a need in the art for a water-borne material which can be used to treat fibrous substrates which improves the strength of such substrates under both ambient as well as wet or humid conditions, and which is non-staining to the substrate, and to a treated substrate which is preferably capable of being repulped and recycled.